This invention relates generally to metal object detection and more particularly to an inductive loop detector suitable for detecting the passage or presence of a vehicle over a defined area of a roadway.
Inductive loop detectors have been widely used for several years in various applications to detect the presence or passage of a vehicle. For example, such detectors have been used in traffic actuated control systems for developing the input data required by a controller to control signal lights. In another typical application, a detector may be connected to a counter which merely functions to accumulate a count of vehicle flow past a certain point.
Various types of inductive loop detectors suitable for traffic applications are known in the prior art. Each detector generally comprises electronic circuitry which operates in conjunction with a loop (i.e. a wire coil) buried in the roadway in a plane substantially parallel to the roadway surface. The circuitry includes components which, together with the loop, form an oscillator whose frequency is dependent on the loop inductance. The loop inductance is in turn dependent on whether or not the loop is loaded by the presence of a vehicle. A vehicle over the loop decreases loop inductance and thus increases the frequency of oscillation. The circuitry monitors the oscillator frequency and generates a "call" (i.e. a vehicle present signal) when a sufficient frequency change is detected. Different techniques have been employed in prior art detectors for monitoring the oscillator frequency but generally, analog circuitry using filters and phase detectors has been employed.